1. Field of the Invention
The present invention generally relates to an image reading apparatus, an image reading method and an image processing apparatus, in which a color image sensor composed of plural line sensors is used.
2. Description of the Related Art
Generally, in an image reading apparatus which can read a color image and an image processing apparatus such as a copying machine and a facsimile, a CCD (charge coupled device) in which line sensors executing photoelectric conversion for hues of RGB are disposed with the same interval among them is installed. For example, a CCD in which three line sensors are disposed with the same interval among them is called as a three line CCD.
When a manuscript (draft) is scanned by using the three line CCD, timing to read a specific position of the manuscript deviates by an offset in each hue. In a case where a color image is read, since there are two types of apparatuses, that is, an apparatus that can process each hue of RGB and an apparatus that can only process by considering all information of RGB after executing correction of each hue (for example, shading correction), it is necessary to correct for the delay amount by some means.
In a conventional image reading apparatus and a conventional image processing apparatus, the image processing is executed so that delay memories, such as FIFO (first-in first-out) memories for pixels of RGB are disposed and delays in each hue of RGB in a line sensor are decreased (for example, refer to Patent Document 1). Hereinafter, delay correction processing in each line is referred to as correction between lines.
In the image reading apparatus and the image processing apparatus, when scaling of an image in the sub scanning direction is executed by changing the scanning speed for the image, the delay amount in each hue is changed. Therefore, it is necessary to change the delay amount by matching the scale factor. For example, in a case where a correction amount between lines is four lines between R and G and four lines between G and B at the time when a 100% image is read, since the scanning speed becomes doubled at the time when the 50% image is read, the delay amount between hues becomes two lines between R and G and two lines between G and B.
In addition, in the image reading apparatus and the image processing apparatus, there are two types, that is, a book read type that reads a manuscript by scanning the manuscript put on a contact glass with the use of an exposing scanning unit, and a sheet through type that reads the manuscript by moving the manuscript without moving the exposing scanning unit. In the image reading apparatus and the image processing apparatus that can realize both the types, since the scanning direction for the manuscript is generally reversed between the types, the relation of the correction between lines in RGB is reversed.
For example, in a case of a condition that G is delayed by four lines for R and B is delayed by eight lines for R in reading by the book read type (hereinafter, referred to as a book read reading), G is delayed by four lines for B and R is delayed by eight lines for B in reading by the sheet through type (hereinafter, referred to as a sheet through reading).
[Patent Document 1] Japanese Laid-Open Patent Application No. 2001-16442
In a case of executing the sheet through reading, the exposing scanning unit (hereinafter, referred to as a carriage) is first moved in the white reference plate direction while the manuscript is moved, correction called shading correction that reduces dispersion in the main scanning direction is executed by reading the white reference plate. In the image reading apparatus and the image processing apparatus, after the white reference plate is read, the carriage is moved to the reading position, and the manuscript is read by moving the manuscript. In the following explanations, this operation is called DF shading correction.
However, in a case of the sheet through reading, since the relation among RGB is reversed from the book read reading, when the DF shading correction is executed in a setting of the correction between lines for the sheet through reading, the correction between lines is reversed, as a result, as shown in FIG. 1, the effective range of the white reference plate is narrowed. FIG. 1 is a conceptual diagram explaining a reason why the effective range of the white reference plate is narrowed.
FIG. 1(a) shows a case in which the DF shading correction is executed in a setting of the correction between lines for the sheet through reading. FIG. 1(b) shows a case in which the DF shading correction is executed in a setting of the correction between lines for the book read reading. A number surrounded by a circle shows each position on a white reference plate 251. The position in the horizontal direction of the number surrounded by the circle shows timing when each pixel of R, G, and B passes through a position.
In this case, since the interval among R, G, and B is four lines, when a pixel of R reads the ninth line, a pixel of B starts to read the white reference plate 251. Further, since data of G are delayed by four lines and data of B are delayed by eight lines for data of R as the correction between lines, timing, when the pixel of B can obtain data that read each position, is delayed by twice the pixel interval. For example, in the case of FIG. 1(a), it is the time when the pixel of R reads the seventeenth line that the DF shading correction is started, and the effective range of the white reference plate 251 is narrowed (DF shading correction possible range in FIG. 1(a)), compared with the case shown in FIG. 1(b). As a result, extra time is required for the DF shading correction.
The extra time consuming for the DF shading correction prevents the reading speed from being high. Shortening the extra time for the DF shading correction is important to realize high speed reading.
In the Patent Document 1, it is described that the setting of the amount of the correction between lines is changed by considering the scaling. However, a problem caused by reading direction difference in the DF shading correction at the time of the sheet through reading is not solved.